终端 header 显示当前在线人数

- src/utils/online_tracker.py：5 分钟滑动窗口，thread-safe 内存追踪
- web/core.py：认证成功时 record_activity(user_id)
- web/routers/ops.py：GET /api/ops/online-users 返回 {online: N}
- 前端 header 每 60s 轮询，Users 图标 + 数字

src/utils/online_tracker.py

@@ -0,0 +1,43 @@
+"""Lightweight online-user tracker.  Records last-seen timestamps in memory
+and exposes a count of users active within a sliding window.  No external
+dependency — just a dict + lock + periodic cleanup thread."""
+
+from __future__ import annotations
+
+import threading
+import time
+from typing import Dict
+
+_online_users: Dict[str, float] = {}
+_lock = threading.Lock()
+_cleanup_interval_sec = 120
+_window_sec = 300  # 5 minutes
+
+
+def _cleanup_stale() -> None:
+    cutoff = time.time() - _window_sec
+    with _lock:
+        stale = [uid for uid, ts in _online_users.items() if ts < cutoff]
+        for uid in stale:
+            del _online_users[uid]
+
+
+def _start_cleanup() -> None:
+    _cleanup_stale()
+    threading.Timer(_cleanup_interval_sec, _start_cleanup).start()
+
+
+_start_cleanup()
+
+
+def record_activity(user_id: str) -> None:
+    if not user_id:
+        return
+    with _lock:
+        _online_users[user_id] = time.time()
+
+
+def online_count() -> int:
+    _cleanup_stale()
+    with _lock:
+        return len(_online_users)